Electro-Harmonix Nano Pog

Polyphonic pitchshifter, in a small box, put into a bigger box. I forgot I did this one. I took a look at this around November 2015 for Moose Electronics who was doing a custom re-house with mods for someone. The pedal had stopped working somewhere between coming out of the original enclosure and going into the larger one he had made for it. I found some pictures on my camera and I actually took some notes, so the details shouldn't be completely hazy.

Work in progress pedal. Wiring was temporary.

ADSP-BF592

Surprisingly, EHX seem to have several different digital designs on the go, using pretty different architectures. They even have multiple polyphonic pitchshifters on the market, with the POG series competing with their Pitchfork. I guess they may be doing some market segmentation and charging more for the established Pog brand. I have also heard that they may be using different designers for different products, which might explain why they use different processors in some products instead of a single unified architecture.

The insides are typical modern EHX, board-mounted jacks, power connector and pots, with a soft-touch momentary switch for bypass. Bypassing is done with a DG419 analog switch, which is a little unusual as they normally use a 3DPT mechanical true bypass (possibly why this was rehoused?) For anyone wanting to convert these, you can short the switch pads and the pedal will always start up in effected mode and stay there. There is a huge programming/debug connector in the top-right.

The Nano Pog has an Analog Devices Blackfin DSP (ADSP-BF592 http://www.analog.com/en/products/processors-dsp/blackfin/adsp-bf592.html#product-overview) doing nearly all of the work. This is a more modern and significantly more powerful part than I usually see, most digital pedals I've looked at recently have used the older Motorala/NXP DSP56k series. The part will run at 200 MHz but I didn't think to look at any clocks on the board (I also didn't have an oscilloscope with any hope of seeing 200MHz signals at the time). AKM AK4552 24bit/96kHz codec is handling conversions.

A TI TPS61620 (http://www.ti.com/product/TPS62120) forms a step-down regulator to get a 3.3V supply from the 9V input. TPS60403 (http://www.ti.com/product/TPS60403) is using as in inverting charge pump in a neat SOT23-5 package. Apart from some opamps (TLC2272C) and an EEPROM that's pretty much it.

This thing had a pretty awful sounding distortion that farted out badly on low notes. The effect could be heard and it tracked correctly, so the digital guts were working correctly. I usually check all for power voltages first, wherever is immediately convenient. The DSP was getting 3.3V. I know what to expect with the opamps, looking there found 9V at V+ and something like 0.5V at V-. Poking about the board found that the charge pump had the same mystery voltage on it's output. The ceramic output cap (C39) is right at the edge of the board and looked like it had been cracked, maybe from taking it out of a tight-fitting enclosure. Not the best layout decision but it probably doesn't matter as most people won't be taking these apart. The datasheet (http://www.ti.com/lit/gpn/tps60403) recommends a 1uF output capacitor, and I had a sample kit of 0805 ceramics, so I tried replacing it. I now had -3.3V at the opamp negative supply pins. I don't usually see +9V and -3.3V supplies like this in pedals. Opamps will work fine with asymmetrical supplies, their outputs will just be limited by them in the usual ways. Pitchshifting should have unity gain, so there's no need for huge output drive. Low voltage charge pumps are easier to find these days too, most won't handle a 9V input. I just don't know why this was chosen over the usual virtual ground/split supply system, as that would allow similar headroom and would eliminate an IC.

Anyway, this was a quick enough fix and it sounded clean afterwards. The polyphonic tracking is cool and it doesn't have much of the shrill digital artifacts you can hear on the octave-up settings on some pitchshifters. I only played with this briefly before handing it back, and although I liked it it didn't exactly inspire many applications. I might need more time with these.

Finished rehouse.

The final version came out great, I think additional A/B outputs and some momentary switching were the custom options.  https://www.facebook.com/mooseelectronicsdublin/